The race to make faster charging, higher capacity, less expensive, and more reliable lithium-ion batteries has been ongoing since they were commercialized by Sony in 1991.[1,2] While modern-day commercial lithium-ion batteries for portable electronic devices primarily employ LiCoO2 cathodes, manganese oxides are more attractive for heavy-duty transportation applications, such as all-electric and hybrid-electric vehicles, because they are abundant, low cost, and environmentally friendly materials.[1,3] In the ideal LiMn2O4 (LMO) spinel crystal structure, the manganese ions alternate between close-packed oxygen planes in a 3:1 ratio, creating a three-dimensional network of interconnected interstitial sites for lithium-ion transport.[4,5] This structure allows for rapid lithiation/delithiation (discharge/charge) reactions that are required for high power applications. Furthermore, LMO offers improved thermal stability relative to LiCoO2, especially in a highly delithiated state, resulting in safer batteries.[6]
However, a major disadvantage of LMO spinel cathodes is that they lose capacity following long term cycling due to Mn2+ dissolution from the surface of the cathode into the electrolyte during charge/discharge as a result of the disproportionation reaction: 2Mn3+→Mn4++Mn2+.[7,8] Researchers have attempted to mitigate Mn3+ dissolution with a number of modifications. One type of modification is to change the composition of the parent LMO electrode by cation substitution (e.g., LiMxMn2-xO4, M=Li, Co, Ni, Zn)[9-14] to reduce the amount of Mn3+ in the structure. An alternative modification is to provide a protective surface oxide coatings, including. Al2O3,[15] ZrO2,[16] Y2O3,[17] and TiO2.[7] For example, Ju et al., disclosed a hybrid graphene/Y2O3/LiMn2O4 microsphere and suppression of the Mn3+ dissolution by the Y2O3 oxide that surrounded the LiMn2O4 core [J. Alloy and Compounds 2014, 584, 454]. In yet another modification, Zhuo et al. disclose the use of liquid polyacrylonitrile (LPAN) to prepare a “graphene-like” membrane as a protective coating that is about 3 nm thick, i.e. approximately one order of magnitude thicker than graphene [J. Power Sources 2014, 247, 721].
Despite attempts to mitigate Mn3+ dissolution, the realization of a thin and uniform surface film that does not compromise surface conductivity remains an outstanding challenge.